1. Field of the Invention
The present invention relates to a semiconductor light-emitting diode, and more specifically, to a semiconductor light-emitting diode having a current diffusion layer.
2. Description of the Related Art
An AlGaInP type material has drawn attention as a material to be used for a light-emitting element which emits light having a wavelength in a range of 550 to 650 nm, since the AlGaInP type material has the largest bandgap of a direct transition type among III-V group compound semiconductor materials excluding a nitride. In particular, a pn-junction type light-emitting diode, in which a light-emitting structure (a layered structure including an active layer) made of an AlGaInP type material lattice-matching with GaAs is grown on a GaAs substrate, is capable of emitting light with higher luminance in a wavelength region corresponding to red to green light, as compared with a light-emitting diode provided with a light-emitting structure made of a material such as GaP or AlGaAs.
In order to form a light-emitting diode with high luminance, it is important to enhance a light-emission efficiency as well as a current injection efficiency into a light-emitting structure, and to allow light to efficiently emit from a device.
A conventional light-emitting diode having a light-emitting structure made of an AlGaInP type material will be described with reference to the drawings. FIG. 8 is a cross-sectional view of such a light-emitting diode 200.
As shown in FIG. 8, a light-emitting diode 200 has a structure in which on an n-type GaAs substrate 61, an n-type GaAs buffer layer 62, a light-emitting structure 69 made of an AlGaInP type material, and a p-type AlxGa1−xAs current diffusion layer 66 are successively layered. The light-emitting structure 69 includes an n-type AlGaInP cladding layer 63, a p-type AlGaInP cladding layer 65, and an AlGaInP active layer 64 interposed between the cladding layers 63 and 65. A p-type electrode 68 is provided on the top surface of the AlxGa1−xAs current diffusion layer 66, and an n-type electrode 67 is provided on the bottom surface of the substrate 61.
A p-type AlxGa1−xAs layer is often used as the current diffusion layer 66 in such a light-emitting diode 200 as described above for the following reason.
The p-type AlxGa1−xAs layer is transparent to light having a wavelength of 550 to 650 nm which can be emitted by the light-emitting structure 69 made of a (AlxGa1−x)yIn1−yP type semiconductor material, and therefore advantageous for obtaining a higher light-emission efficiency. Furthermore, the p-type AlxGa1−xAs layer has a low resistivity, which makes it easy to obtain an ohmic contact with the p-type electrode 68 when employed as the current diffusion layer 66. In addition, it is easy to grow a p-type AlxGa1−xAs layer including crystal of a higher quality, as compared with an (AlxG1−x)yIn1−yP type semiconductor material. Thus, the p-type AlxGa1−xAs layer can be relatively easily grown after the growth of a double hetero layer (the “DH layer”) made of an (AlxG1−x)yIn1−yP type, i.e., the light-emitting structure 69.
Regarding the material to be used for the current diffusion layer 66, comparisons between a conventional AlxGa1−xAs type material and an (AlxGa1−x)yIn1−yP type material will be explained below. Throughout the present specification, the term “Al mole fraction” refers to a mole fraction x of Al with respect to Ga (i.e., x=Al/(Al+Ga)). The term “In mole fraction” refers to a mole fraction 1−y of In with respect to Al and Ga (i.e., 1−y=In/(Al+Ga+In)). Moreover, the compositions of “(AlxGa1−x)yIn1−yP” and “AlxGa1−xAs” may be simply referred to as “AlGaInP” and “AlGaAs”, respectively.
FIG. 9 is a graph showing the relationship between the resistivity of an AlxGa1−xAs current diffusion layer lattice-matching with a GaAs substrate and the Al mole fraction x thereof, and between the resistivity of an (AlxGa1−x)0.51In0.49P current diffusion layer (i.e., 1−y=0.49) lattice-matching with the GaAs substrate and the Al mole fraction x thereof.
It is understood from FIG. 9 that the AlxGa1−xAs current diffusion layer exhibits a resistivity of about 0.06 Ω cm, for example, at an Al mole fraction x of 0.8. Thus, a low resistivity can be obtained even at a high Al mole fraction x.
In contrast, the (AlxGa1−x)0.51In0.49P current diffusion layer exhibits a resistivity of about 0.15 to about 3 Ω cm at an Al mole fraction x in the range of 0 to 0.8. These values of resistivity are larger by one order of magnitude than those obtainable with the AlxGa1−xAs layer. Even if the Al mole fraction is decreased, the resistivity is still higher by 50 times than that of the AlxGa1−xAs layer. Accordingly, the (AlxGa1−x)0.51In0.49P current diffusion layer is inferior to the AlxGa1−xAs current diffusion layer, since a low resistivity cannot be obtained.
Furthermore, in order for the (AlxGa1−x0.51In0.49P current diffusion layer to allow light having a wavelength of 550 to 650 nm emitted from the light-emitting structure 69 to transmit therethrough, it is required to prescribe the Al mole fraction x to be 0.50 or more. In this case, the resistivity of the (AlxGa1−x)0.51 In0.49P current diffusion layer becomes higher by two orders of magnitude, as compared with that of the AlxGa1−xAs current diffusion layer.
If the resistivity is high, the current diffusion ability of the current diffusion layer is decreased, and a current does not spread over the entire chip. As a result, light-emission from a portion of the light-emitting structure right below the electrode becomes dominant. The light emitted from such a portion is likely to be blocked by the electrode, whereby the emitted light is unlikely to be output. Accordingly, the increase in resistivity of the current diffusion layer causes a light-emission efficiency to decrease. Furthermore, the increase in resistivity of the current diffusion layer causes an operating voltage to increase.
Thus, the (AlxGa1−x)0.51In0.49P current diffusion layer which is lattice-matched with GaAs has a higher resistivity than that of the AlxGa1−xAs current diffusion layer, and consequently has adverse effects on the operational characteristics of a resultant light-emitting diode. Therefore, the AlxGa1−xAs layer is typically employed as the current diffusion layer in the conventional art, instead of the AlGaInP type layer.
As described above, the AlxGa1−xAs layer suffices as the current diffusion layer of a light-emitting diode, as far as the resistivity is concerned. In order for the AlxGa1−xAs current diffusion layer to be transparent with respect to light having a wavelength of 550 to 650 nm, it is required to prescribe an Al mole fraction x thereof to be 0.65 or more. However, when the Al mole fraction x becomes high, the AlxGa1−xAs layer will exhibit a deliquescence. Thus, in the case where a light-emitting diode having an AlxGa1−xAs layer with a high Al mole fraction x is operated under the conditions of high temperature and high humidity, light intensity is likely to be remarkably decreased.
FIG. 10 shows changes in a chip light intensity (i.e., an intensity of light obtained from the semiconductor light-emitting diode chip) with a passage of time, in the case where a light-emitting diode chip having the AlxGa1−xAs current diffusion layer is operated under the conditions of a temperature of 60° C. and a humidity of 95%. In FIG. 10, data for the chip light intensities are indicated as relative values.
As seen from FIG. 10, as an operating time becomes longer, a chip light intensity is decreased. Furthermore, as an Al mole fraction becomes larger, a chip light intensity is more remarkably decreased.
Such a deterioration of a light-emitting diode will be described with reference to FIG. 11. FIG. 11 shows the light-emitting diode 200 previously described with reference to FIG. 8, but in a deteriorated condition. Since like components are designated with like reference numerals, the explanations thereof are omitted here.
As shown in FIG. 11, while operating the light-emitting diode 200 under the conditions of high temperature and high humidity, the surface of the AlGaAs current diffusion layer 66 with a high Al mole fraction tends to absorb moisture so as to be deliquescent, thereby resulting in black-colored portions 66a on the surface thereof. Such black-colored portions 66a on the surface of the current diffusion layer 66 absorb the light (represented by arrows in FIG. 11) emitted from the inside of the light-emitting diode 200. Thus, in the case where the AlGaAs layer with a high Al mole fraction is employed as the current diffusion layer, it is difficult to provide a light-emitting diode exhibiting stable luminance over a long period of time.
As described above, although the AlGaAs layer has been typically used as the current diffusion layer in the conventional semiconductor light-emitting diode for the reason that a low resistivity can be obtained, the AlGaAs layer is not reliable under the conditions of high temperature and high humidity. On the other hand, when the (AlxGa1−x)0.51In0.49P layer capable of lattice-matching with the GaAs substrate typically used is employed as the current diffusion layer in place of the AlGaAs layer, the resultant current diffusion layer will then have a higher resistivity, so that sufficient luminance cannot be obtained.